Vibrational self-quenching rates of CO(v=1,2) measured via transient diode laser absorption spectroscopy

Abstract

New laboratory measurements of the rate coefficients for the vibrational relaxation of CO(v=1) and CO(v=2) through collisions with CO have been performed. The experimental approach involved creating a temperature-jump in an equilibrium mixture of CO and bath gases by photolyzing a trace amount of O3 in the mixture with a pulsed UV laser. This shifted a small fraction of the CO population to excited vibrational states. Transient diode laser absorption spectroscopy was then used to observe the evolving vibrational state populations as a function of time. Rate coefficients were deduced from the change in collisional relaxation rate with quencher concentration. This work was motivated by the need for a robust non-LTE model to describe Titan's upper atmosphere. Because the gas collision frequencies are not sufficient to reach thermal equilibrium in a non-LTE environment, the distribution of states is not easily predicted. To properly model such a region, all contributing vibrational energy transfer mechanisms must be thoroughly characterized. Despite their importance, the uncertainties associated with several CO quenching rate parameters in the literature are large. In this work, the rate coefficient for CO(v=2) self-quenching was measured to be (2.7 ± 0.6) x 10−12 cm3 molecule−1 s−1and the overall rate coefficient for CO(v=1) self-quenching was measured to be (1.8 ± 0.3) x 10−12 cm3 molecule−1 s−1. Each represents a significant decrease in uncertainty compared with values being used in current Titan non-LTE models.